This paper presents a modification of the behaviour-based individuals model of Oystercatchers on the Exe Estuary, England which was adapted to the Seine estuary for three species of waders: Eurasian Oystercatcher, Eurasian Curlew and Dunlin. The purpose of this model was to analyse, in the future, the impact of the construction of Port 2000 at le Harvre on the mortality and body condition of these species. The aim of the present paper is to present a modelling approach that may be useful for guiding policy on a wide range of environment management issues involving estuarine migratory birds. The present preliminary model predicted quite well the distribution of the three species within the estuary and there was also good agreement between the predicted numbers of the three species feeding on mudflat 4 (which will disappear with the construction of Port 2000) and the numbers actually recorded during fieldwork. Results from this first version of the model suggest that, at the present time, the numbers, mortality and body condition of only the Dunlin seems to be density-dependent. These very provisional results therefore suggest that Dunlin may be the only one of the three species to be affected by the construction of Port 2000. These results are not definitive and more field data are required if more reliable evaluations are to be made of the impact of building the Port and of the efficiency of the proposed mitigation measures.